php 数学运算验证码实现代码

代码如下:
<?php
//-------------------------------------
// 文件说明：数学运算验证码
// 文件作者：Jesse Lee
// 最后更新：2008-09-07
//-------------------------------------

session_start();

$sessionvar = 'vdcode'; //Session变量名称
$width = 150; //图像宽度
$height = 20; //图像高度

$operator = '+-*'; //运算符

$code = array();
$code[] = mt_rand(1,9);
$code[] = $operator{mt_rand(0,2)};
$code[] = mt_rand(1,9);
$code[] = $operator{mt_rand(0,2)};
$code[] = mt_rand(1,9);
$codestr = implode('',$code);
eval("\$result = ".implode('',$code).";");
$code[] = '=';

$_SESSION[$sessionvar] = $result;

$img = ImageCreate($width,$height);
ImageColorAllocate($img, mt_rand(230,250), mt_rand(230,250), mt_rand(230,250));
$color = ImageColorAllocate($img, 0, 0, 0);

$offset = 0;
foreach ($code as $char) {
$offset += 20;
$txtcolor = ImageColorAllocate($img, mt_rand(0,255), mt_rand(0,150), mt_rand(0,255));
ImageChar($img, mt_rand(3,5), $offset, mt_rand(1,5), $char, $txtcolor);
}

for ($i=0; $i<100; $i++) {
$pxcolor = ImageColorAllocate($img, mt_rand(0,255), mt_rand(0,255), mt_rand(0,255));
ImageSetPixel($img, mt_rand(0,$width), mt_rand(0,$height), $pxcolor);
}

header('Content-type: image/png');
ImagePng($img);
?>